Method and Apparatus for Pasteurizing Shell Eggs Using Radio Frequency Heating

ABSTRACT

Radio frequency (RF) energy is used to quickly heat the yolk portion of a shell egg. An anode and corresponding cathode are applied to each individual egg. As the egg is selectively and systematically rotated, RF energy and a stream of cooling fluid (preferably water) are simultaneously applied to the egg. This initiates pasteurization of the egg yolk while maintaining a low temperature in the heat-sensitive albumen (egg white) thus preventing denaturation of the albumen. Immediately after the RF yolk heating process, the egg is placed in a hot water bath designed to rapidly pasteurize the albumen as well as to minimize heat loss from the yolk and pasteurize any portion of the yolk that is not already pasteurized through the RF yolk heating process.

FIELD OF THE INVENTION

The disclosed method and apparatus relates to pasteurizing shell eggs.Specifically, the method and apparatus described herein relates to atleast partially pasteurizing egg yolks with radio frequency (RF) heatingand then completing the yolk pasteurization process as well aspasteurizing the egg whites (i.e. the “albumen”) using a variation ofconventional methods.

BACKGROUND OF THE INVENTION

Nearly 200 million “shell eggs” are consumed in the United States (US)each day. “Shell eggs” are non-powdered conventional eggs that arenaturally produced by hens. Shell eggs are among the most nutritiousfoods on earth and can be part of a healthy diet. However, someunbroken, clean, fresh shell eggs may contain bacteria that can causefoodborne illness. While the number of eggs affected is quite small, 30%of the US population is highly susceptible to bacteria that may be foundin eggs. Pregnant women, infants and young children, the elderly, andthe immunocompromised are particularly at risk. Shell eggs topped thelist of “Riskiest Federal Drug Administration -Regulated Foods” and hadthe most documented outbreaks from 1990 to 2006. The US Department ofAgriculture estimates that pasteurization of all shell eggs in the USwould reduce the annual number of illnesses by more than 110,000.

Currently only two companies in the US pasteurize shell eggs. Bothcompanies use a hot water immersion process whereby the eggs aresubmerged in hot (approximately 56.7° C.) water for about 60 minutes.While long exposure to high temperatures is required to pasteurize theegg yolk, the high temperatures damage and degrade the quality of thealbumen. Albumen damage can occur when the albumen is subjected to atemperature of 57° C., or held at 49° C. for 60 min. The albumen damageis manifested by varying amounts of coagulation and/or denaturation. Bycontrast, egg yolk can withstand temperatures of 64° C., or more,without being damaged.

Attempts have also been made to pasteurize eggs using microwave-basedprocesses and at least one prior art patent discloses the use of radiofrequency (RF) energy to pasteurize shell eggs. However these prior artprocesses proved to be inefficient and damaging to the targeted eggs.

Penetration depth of electromagnetic energy increases as the frequencydecreases. Thus RF energy, in the range of 10 to 100 MHz can have apenetration depth ten times greater than that of microwave energy (2.45GHz). Consequently, the deeper penetration of RF energy should heat theyolk better than microwave energy.

The current inventors' method and apparatus uses RF energy to directlyand preferentially heat the egg yolk as the targeted egg is rotated,rather than to first heat the albumen and then the yolk (as isconventionally done by conduction heating a non-rotating egg). By usingRF energy to heat the egg yolk directly, the process described herein ismore efficient and avoids damage to the albumen. The inventors' methodand apparatus quickly heats the yolk to inactivate pathogenic bacteriausing RF energy. After the yolk reaches pasteurization temperature, thealbumen is rapidly pasteurized using conventional methods. The neteffect of the inventor's disclosed method and apparatus is to rapidlypasteurize shell eggs with minimal damage to quality and significantsavings in time and other resources.

SUMMARY OF THE INVENTION

This disclosure is directed to a system for pasteurizing shell eggs. Thesystem includes an egg rotating assembly structured to rotate the egg,and at least one electrode that is in contact with the egg. The systemis structured so that, as the rotating assembly rotates the egg, radiofrequency energy is directed to the egg to pasteurize at least a portionof the egg.

This disclosure is also directed to a method of pasteurizing a shellegg. In accordance with the method, at least one electrode is placed incontact with a rotating egg. As the egg rotates, radio frequency energyis applied to the egg to pasteurize at least a portion of the egg.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of the inventor's preferred pasteurizingapparatus.

FIG. 2 is a schematic view of an alternative embodiment of thepasteurizing apparatus shown in FIG. 1.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

As discussed above, the presence of bacteria (particularly Salmonellaenteritidis) is a health concern for consumers of shelleggs—particularly uncooked shell eggs. In contaminated eggs, theSalmonella bacteria are often found in the egg yolk. While prior artmethods pasteurize egg yolks by applying heat to the outside of theeggs, the method and apparatus described herein at least partiallypasteurizes egg yolks by focusing heat on the center (including theyolk) of the eggs, which is both efficient and effective at destroyingany harmful bacteria present.

As used herein, the terms “pasteurization” and “pasteurize(d)” refer totreatments sufficient to kill pathogenic microorganisms contained withinthe shell egg being treated and thereby eliminate the threat of consumerexposure to effective amounts of (for example) Salmonella.Pasteurization methods cause about a five log cycle (99.999%) reductionof Salmonella bacteria.

Additionally, as used herein, the phrase “at least partially pasteurizesa portion of the egg” means that the temperature of at least a portionof the egg is raised to a pasteurizing temperature so that pathogenicmicroorganisms within that portion of the egg are killed. In thepreferred embodiment, a portion of the egg that is at least partiallypasteurized by RF energy is within the egg yolk.

The preferred embodiment of the inventors' pasteurizing apparatus 10 isgenerally shown in FIG. 1. The apparatus 10 includes an active electrode18 and a grounded electrode 19 disposed on opposite sides of the egg 14,and two cooling fluid applicators 12 that direct cooling fluid(preferably water) onto a target egg 14. Specifically, tepid water isdirected onto the egg 14 during the initial pasteurization process. Inthe preferred embodiment, the tepid water is about 35° C., but may be inthe range of 25 to 45° C., although broader ranges may also befunctional and should be considered within the scope of the invention.In an alternative embodiment, the entire apparatus 10 is submerged in awater bath during and/or after the initial pasteurization process.

Note that, during the yolk RF heating process, water is applied to theouter shell of the egg 14 to cool the egg 14, whereas, during thealbumen pasteurization process (and conventionally), water is applied tothe outer shell to heat the egg 14 albumen.

As the water runs over the egg 14, the egg 14 is rotated by a rotatingassembly 20. The rotating assembly 20 comprises at least a pair ofrollers 16 which slowly rotate the egg 14, preferably at about 15revolutions per minute, however the rotation speed may vary and may bein the range of 5 to 25 revolutions per minute. In the preferredembodiment, the roller assembly 20 includes at least one electricalmotor in communication with the rollers 16. In additional alternativeembodiments, the roller assembly 20 may include any rotating means anddrive mechanisms known in the art.

In alternative embodiments, the electrodes 18, 19 may be rotated arounda (non-rotating) stationary egg 14 and thereby achieve a similar effect.In further alternative embodiments, more than one pair of electrodes maybe deployed around the circumference of the egg 14. In this embodiment,the electrodes emit RF energy in a predetermined pattern so thatalthough the egg 14 and the electrode apparatus is stationary, RF energyis emitted from positions around the circumference of the egg 14, andthereby achieves an effect somewhat similar to the effect describedabove. For the purposes of simplicity and clarity, only one (midsized)active electrode 18 and one grounded electrode 19 are shown in schematicFIGS. 1 and 2. However embodiments incorporating more than one pair ofelectrodes 18, 19 and varying sizes and shapes of electrodes 18, 19should be considered within the scope of the invention.

In further alternative embodiments, sensors 17 (see FIG. 2) associatedwith the electrodes 18, 19 monitor temperature and other parameters(such as resistance, inductance, etc.) of the egg 14 and rotate or applyRF energy (through the electrodes 18,19) to the egg 14 based on sensor17 indications of the conditions within the egg 14. Specifically, one orboth of the RF and the rotating assemblies are at least partiallycontrollable by a sensor measuring directly or indirectly at least oneproperty of the egg. There are multiple types of sensors 17 that (incommunication with a controller) are capable of making determinationsregarding the conditions (such as temperature, resistance, etc.) withinthe egg. For the purposes of simplicity and clarity, only one (midsized)sensor 17 is shown in schematic in FIG. 2. However embodimentsincorporating more than one sensor 17 should be considered within thescope of the invention.

In the preferred embodiment, about 75 watts of RF energy is applied tothe egg 14 through the electrodes 18, 19, which are positioned in thecenter of the egg 14 on opposing sides of the egg 14. In alternativeembodiments, RF energy in the range of 5 to 200 watts may be applied.The 75 watts of RF energy is maintained preferably for about 5 minutes,however in alternative embodiments, the power may be maintained for atime ranging between 2 and 10 minutes, dependent on the size of the egg14 and other factors associated with both the egg 14 and the apparatus10.

The intensity and duration of the treatment may also be varied. Forexample, a power of 100 watts may be maintained for 2 minutes, followedby 75 watt power for one minute, and 50 watt power for one minute. TheRF energy is preferably applied at 60 MHz, however in alternativeembodiments, the energy may be applied at a frequency in the range of 1to 100 MHz. In alternative embodiments, as described above, thetreatments may also be varied based on sensor 17 indications of otherparameters associated with the egg 14.

As shown in FIG. 1, in the preferred embodiment, the active electrode 18and grounded electrode 19 are generally in the form of electricallyconductive brushes that communicate energy through the moistened surfaceof the egg 14. The optimal size of the electrode brushes 18, 19 areabout 60% of the (end to end) length of the egg. Directing theapplicators 12 to apply the water to the brush-egg contact pointimproves the RF energy transfer from the brushes 18, 19 to the egg 14.The electrodes 18, 19 may alternatively be comprised of a copper mesh orother conductive materials. The electrodes 18, 19 are held in contactwith the egg 14 by a pair of weighted arms 22. Alternatively theelectrodes 18, 19 may be held in place to maintain contact with the egg14 by a clamp, a spring, a retaining ring, or other means.

In an alternative embodiment (such as FIG. 2), the active electrode 18is held in contact with the egg 14 by a single weighted arm 22, and thegrounded electrode 19 forms at least a portion of an outer surface ofone of the rollers 16. In another alternative embodiment, either one orboth of the active electrode 18 and/or ground electrode 19 may comprisea weighted arm device and/or a portion of one or both of the rollers 16,or another portion of the roller assembly 20. In further alternativeembodiments, the electrodes 18, 19 comprise an electrical conductanceassembly that may contact the surface of the egg 14 by any means knownin the art.

In operation, the individual devices described herein may be replicatedmultiple times so that the pasteurizing operation is “scaled up” forcommercial production. Multiple examples of the apparatuses shown inFIG. 1 and/or FIG. 2 may be aggregated to form a pasteurizing array sothat large numbers of eggs are pasteurized in a single commercialoperation.

As the RF energy is applied to the egg 14, the temperature of the eggyolk is increased to preferably about 59° C., and the temperature of thealbumen is maintained below 57° C. In alternative embodiments, thetemperature of the yolk may be elevated to a temperature in the range of57 to 60° C. At 60° C., 99.999% of the bacteria within the egg yolk arekilled within about 3 minutes.

In the preferred embodiment, after the yolk is at least partiallypasteurized, the egg 14 is immediately removed from the apparatus 10 andsubjected to a hot water bath process. Specifically, the egg 14 isplaced in a conventional hot (56 to 57° C.) water bath for about 20minutes. In alternative embodiments the hot water may be applied from 7to 25 minutes. Using this method, the albumen is heated to approximately56.7° C. within about 5 minutes. Once the temperature of the albumenreaches 56.7° C., it takes about 2 additional minutes to kill 99.999% ofany bacteria. In addition to heating the albumen, the hot water bathprocess also minimizes heat loss from the yolk and pasteurizes anyportion of the yolk that is not already pasteurized through the RFheating process.

In alternative embodiments, following RF heating, the egg 14 may beretained in the RF heating apparatus 10 (as shown in FIG. 1 or 2) andthe egg 14 and apparatus 10 may simply be submerged in a hot water bathuntil the egg 14 is pasteurized.

In further alternative embodiments, following the egg yolk heatingprocess, the temperature of the water applied by the applicator tube(s)12 may be significantly increased so that hot (rather than tepid) wateris applied to the egg 14. The albumen is then pasteurized by theapplication of the hot water from the applicator(s) 12 rather than froma water bath. In even further alternative embodiments, following the RFyolk heating process, the egg 14 may be placed in a moist hot airenvironment to complete pasteurization.

EXAMPLES

Escherichia coli (ATCC 35218) was maintained on tryptic soy agar (TSA;Becton, Dickinson and Company, Sparks, Md.) at 4° C. The RF research wasperformed in a food pilot plant, so Salmonella could not be used;however, E. coli (ATCC 35218) has been determined to have slightlygreater thermal resistance than Salmonella. The E. coli was cultured intryptic soy broth (Becton, Dickinson and Company) with shaking at 37° C.for 16-18 hours.

Shell eggs were obtained from a local commercial egg producer. Eggs weresorted to obtain eggs weighing 57 to 61 grams each and were storedovernight at room temperature (23 C) prior to being inoculated. Thelarge ends of the eggs were first perforated by hand with an 18 gaugesterile needle. Following shell perforation, eggs (with the large endup) were placed in an inoculation device (designed and assembledin-house) that consistently injected E. coli culture into the centers ofthe yolks. The device was composed of a Hamilton, Gastight model 1725LT,luer tip, autoclavable, 250 μl glass syringe and a 16 gauge, 3.8 cmneedle that were clamped to a low-speed actuator.

The glass syringe was filled with stationary phase E. coli culture andcentered over the hole in the large end of the egg. The actuator slowlypushed the tip of the needle through the hole to a depth of 3.2 cm andinto the center of the yolk. Eggs were then slowly injected with 50 μlof inoculum. This was followed by a 30 second waiting period to allowpressure equilibration within the egg to prevent inoculum leakage fromthe yolk. The actuator then slowly retracted the needle and the egg holewas sealed with a drop of fast-curing epoxy gel and allowed to cure forat least 30 min before pasteurization treatments.

Preliminary trials using a dye technique were done to confirm thatcultures were inoculated into the geometric center of the yolks. Thistechnique consisted of injecting 50 μl of dye into the egg followed by astandard hard-boiling procedure to demonstrate consistent placement ofthe dye near the center of the yolk with no detectable drift. Some eggswere inoculated with 50 μl of E. coli culture at a depth of 1 cm(instead of 3.2 cm) to study inactivation in the albumen. In both theyolk and albumen inoculations, the population of E. coli in the egg wasapproximately 7 log cfu/ml (control).

Following RF treatment, eggs were sampled by first aseptically crackingthe contents into a stainless steel Waring Mini-Sample Blender Container(model MC2, Waring Products, Torrington, Conn.), and blending on mediumspeed with a Waring model LB 10G variable speed blender for 1 minute.The contents were then serially diluted with sterile 0.1% peptonesolution and plated onto Petrifilm Aerobic Count Plates (3M, St. Paul,Minn.). Plates were incubated at 37° C. for 24 h before enumeration.

Some eggs were not inoculated and were used in studies to determine thetemperatures of the albumen and yolk following RF treatment. The contentof an egg was placed in a Petri dish and the temperatures of the albumenand yolk were measured with a type K thermocouple as well as theappearance of the albumin and yolk was determined.

Example 1

An inoculated egg was placed on rollers (as shown in FIG. 1). Theelectrode brushes were placed in contact with the egg using the weightedarms. Using a motor connected to the rollers, the rotational speed ofthe rollers was adjusted to 15 RPM. Tepid (35° C.) water was directedbetween the egg and the electrode brushes. The purpose of the water wasto improve the coupling of RF energy to the egg and to cool the eggshell and protect the albumen from overheating. Thus, the RF energypreferentially heated the yolk.

The electrodes were then connected via a coaxial cable to a RF powersupply (ModCPS1000/60, Comdel, Gloucester, Mass.). The power supplyproduced up to 1 kilowatt at a frequency of 60 MHz and an outputimpedance of 50 Ω. An impedance matching network was designed into theRF energy applicator's circuit to maintain 50 Ω to ensure maximumcoupling of energy from the power supply. The power supply includedinstrumentation that measured forward and reflected power. Because ofexcellent matching, the reflected power was never more than 5 watts. Theexperimental procedure consisted of two steps. In the first step, RFenergy was used to preferentially heat the yolk by applying 75 watts toan egg for 6 minutes

In the second and final step, hot water was used to heat the shell andalbumen to quickly inactivate bacteria that may reside there. For thispurpose, the egg was disconnected from the rotating apparatus and placedin a water bath (WB) at 56.7° C. for 20 min. The temperature of the WBwas measured with a type K thermocouple connected to a data logger(model HH309A, OMEGA Engineering, Stamford, Conn.).

For the case where the yolk was inoculated and the two-step (RF+WB)26-minute process was applied, the population of E. coli was reduced by6.3 log and no damage to the albumen, yolk, or any other part of the eggoccurred.

When shell eggs were processed using only a 20 minutes hot waterimmersion (WB only), the bacterial reduction was 2.2 log and there wasno visible damage. When the WB only treatment was extended to 60minutes, the inactivation increased to 5.8 log, but the albumen had ahazy appearance.

Example 2

For treatment with RF, an applicator electrode was attached to the largeend of the egg and another to the small end. The electrodes consisted ofcopper mesh (woven wire cloth, wire diameter 0.028 cm, open area 67.9%)that was held in place with a retaining ring of zinc plated steel (for4.1 cm shaft diameter).

A mesh was selected because the wire structure provided excellentelectrical conductivity while the spaces between the wires allowed heatto freely transfer from the shell outwards, thus preventing anylocalized hot spots directly below the shell in the albumen. A clamp,made of high-strength electrical insulating material applied a slightforce to the electrodes which resulted in good contact between theelectrodes and the egg. The egg and applicator electrodes were thencovered with 35° C. deionized water by placing them in a 2.5 1plastic-walled water bath. The water cools the egg shell and protectsthe albumen from overheating.

The experimental procedure consisted of two steps. In the first step, RFenergy was used to preferentially heat the yolk by applying 50 watts toan egg for 30 seconds, rotating the egg 180 degrees (around the axisrunning from the large end to the small end) for 10 seconds to improveheating uniformity, applying an additional 50 watts for 30 seconds,rotating 90 degrees for 10 seconds, applying 25 watts for 45 seconds,rotating 180 degrees for 10 seconds, and finally applying an additional25 watts for 45 seconds.

The power was reduced in the latter segments to further preventoverheating and was compensated for by increasing the treatment times.In the second step, the egg was disconnected from the applicatorelectrodes and placed in a WB at 56.7° C. for 20 min.

For the case where the yolk was inoculated, the population of E. coliwas reduced by 5.1 log. No damage was observed. The maximum temperaturesof the albumen and yolk were 49 and 59° C., respectively, after the 3minutes RF treatment (end of first step). The entire process (RF+WB) was23 minutes. When the RF treatment was increased to 4 minutes, theinactivation increased to 6.4 log, but the albumen appeared slightlyhazy.

For the case where the albumen was inoculated (as opposed to the yolk)and the two-step (RF+WB) process was applied, the population of E. coliwas reduced by 5.0 log. Thus, the two-step process outlined herein iscapable of pasteurizing shell eggs independent of where the bacteria mayreside.

For the foregoing reasons, it is clear that the method and apparatusdescribed herein provides an innovative method and apparatus forpasteurizing shell eggs. The current system may be modified in multipleways and applied in various technological applications. The disclosedmethod and apparatus may be modified and customized as required by aspecific operation or application, and the individual components may bemodified and defined, as required, to achieve the desired result.

Although the materials of construction are not described, they mayinclude a variety of compositions consistent with the function describedherein. Such variations are not to be regarded as a departure from thespirit and scope of this disclosure, and all such modifications as wouldbe obvious to one skilled in the art are intended to be included withinthe scope of the following claims.

1-22. (canceled)
 23. A method of pasteurizing a shell egg comprising thesteps of: (a) positioning at least one electrode to contact the egg, theelectrode generating RF energy; (b) a rotating assembly comprising aplurality of rollers causing relative rotation between the at least oneelectrode and the egg; (c) directing the RF energy to the egg to atleast partially pasteurize a portion of the egg.
 24. The method of claim23 further comprising: (d) directing a stream of cooling fluid over theegg as the RF energy is applied to the egg.
 25. The method of claim 24further comprising: (e) exposing an outer shell of the egg to hot wateror moist hot air until the egg is pasteurized.
 26. The method of claim25 wherein, in step (e), the egg is exposed to the hot water byimmersing the egg is a hot water bath.
 27. The method of claim 23wherein, in step (c) the portion of the egg that is pasteurized is inthe yolk.
 28. The method of claim 23 wherein, in step (a), the at leastone electrode comprises at least a pair of electrodes.
 29. The method ofclaim 28 wherein the at least a pair of electrodes comprises a pair ofbrushes.
 30. (canceled)
 31. The method of claim 25 wherein the egg isexposed to the hot water by a fluid applicator that directs the hotwater over the egg.
 32. The method of claim 23 wherein, in step (b), therotating assembly is structured so that the rotating assembly rotatesthe egg at a speed in the range of 5 to 25 revolutions per minute. 33.The method of claim 23 wherein, in step (a), the electrode is structuredso that the RF energy applied to the egg is in the range of 5 to 200watts.
 34. The system of claim 23 wherein, in step (a) the electrode isstructured so that the RF energy is applied to the egg for a time periodin the range of 2 to 10 minutes.
 35. The system of claim 23 wherein, instep (a), the RF energy applied to the egg is in the frequency range of1 to 100 Mhz.
 36. The system of claim 23 wherein, in step (b), therotating assembly is structured so that the egg remains stationary andnon-rotating, and the rotating assembly physically rotates theelectrodes at least one electrode around the egg.